Updated Monday Morning: See the latest rain timeline here
There is yet another large storm in what has been an active pattern over the past month dominating the southern US. It is mimicking the El Nino expectations for winter, but the winter pattern is not here yet. What I am interested in is how this rain event will push north and run into a large block of dry air. The same air mass that has brought back the chill today. You might remember about a week or two ago when we expected rain that kept getting shaved away from north to south due to something similar. Here we go again, and I have two computer model with two different rain plots. So you might have seen varying forecasts from your sources on TV or apps having your confused.
The NOAA rainfall outlook does show much of our area with 0.50″ or more, with a drop off north of Baltimore and northeast due to that dry air. That is the pattern that will deflect this storm south off the Mid Atlantic coast. The trend has been for storms to stay farther south and east than expectations, something I attribute to the El Nino upper level winds not being picked up by computer models.
If you are plotting along, note this: The exact track of this storm may be similar to the past month and set up some form of atmospheric memory. However, in the winter, systems will be stronger and could adjust north or south. The important thing for now is to identify the bias in the models (trend for systems to stay south and east of main track nearby) and the frequency there is a blocking High in the northeast that erodes away the leading edge of rain. In the winter, this could make or break the onset time of wintry precipitation, and for that matter event totals. In other words, the southern edge keeps winning and the northern edge under achieves. Less is more… likely to happen.
The GFS Model gets ridiculed by many meteorologists, but it was best with the short cut off of rain in our last event. Here it shows the influence of the dry air, and actually shoves the main storm well south of us again, thus keeping Monday dry. It is an upper level feature that brings in the rain Tuesday morning. I have to lean with the less aggressive outlook here. The NAM Model (shown below) in it’s lower resolution form, is known for overdoing events. This brings in the rain unimpeded Monday afternoon and into Tuesday. More rain with that scenario, but ending sooner. This model has been way off in recent weeks and no reason to think otherwise now. So this may be why some forecasts have the rain arriving sooner. If I am wrong about this, I will be the first to point it out and refer back to this post.